The present invention relates generally to the fields of spectroscopy and of calibrating a spectroscopic device, such as a spectrometer for predicting analyte levels. More particularly, the present invention relates to a method of optimizing wavelength calibration to facilitate the transfer of a calibration model from a primary device to a secondary device.
Spectroscopy is based on the analysis of how incident radiation interacts with the vibrational and rotational states of molecules, often within a matrix such as blood or living tissue, which are of analytical interest. Spectrometers and other similar devices that use spectroscopic measurement techniques have gained increased popularity because of the ability to provide fast and non-invasive measurements of concentrations of different chemicals or analytes. In particular, spectrophotometry is a type of spectroscopy commonly used to quantitatively measure properties such as analyte concentration(s) based on spectral energy distribution in the absorption spectrum of a sample solution or medium. In spectrophotometry, the energy distribution is typically analyzed within a range of the visible, ultraviolet, infrared, or near-infrared spectra. For example, near-infrared radiation (NIR) is electromagnetic radiation having a wavelength between about 750 and 2500 nanometers (nm). Near-infrared spectrophotometry generally uses instruments with quartz prisms in monochromators and with lead sulfide photoconductor cells or photodiodes as detectors to observe absorption bands. Near-infrared spectrophotometry is, for example, increasingly being used to measure in vivo analytes such as glucose, fructose, glycerol, and ethanol.
Spectroscopic devices are well known in the art and are described in detail, for instance, in U.S. Pat. Nos. 5,361,758 and 5,771,094, the contents of which are incorporated herein by virtue of this reference. In general, a typical spectrometer system includes a light source which is projected through the sample to be examined, a sample interface mechanism, a spectrometer to separate the light into its component wavelengths, a detector, amplification electronics, and a microprocessor or computer system. By measuring the loss (absorption), between the source and the detector and applying appropriate chemometric or mathematical techniques, it is possible to determine the chemical analytes being examined since different chemicals absorb different amounts of light. The detector or photodetector generally includes a photodiode array of pixels enabling the detector to simultaneously detect the intensities of a number of different spectral components at distinct wavelengths. The intensities at these distinct wavelengths can be used to predict, in turn, the quantities or concentrations of the analyte(s) of interest.
Calibration of spectrometers and of analytical instruments in general is necessary to ensure the accuracy of measurements performed by such devices. In essence, calibration is the development of a model or algorithm that predicts the properties (e.g. analyte concentrations) of a sample from the spectrometer""s response. To calibrate a spectrometer, the spectral response of several calibration samples or standards having known concentrations of an analyte of interest is measured. By combining the known concentration data with the measured spectral response data, a calibration model (i.e. a mathematical relationship) can be developed using a xe2x80x9cbest fitxe2x80x9d regression technique (e.g. partial least squares or PLS) between the spectral measurements and analyte or property of interest. The calibration model or algorithm is then stored in a non-volatile memory, such as for example, in a microprocessor system of the device. In most cases, the spectrometer""s response is a measure of a number of variables, e.g. a number of different chemical species present in a sample, and so calibration is based on a multivariate calibration model. The spectrometer and its calibration model can then be used to estimate the property or properties (e.g. analyte concentrations) of an unknown sample. By detecting the pixel position of a spectral component measured by the spectrometer, the wavelength of that spectral component is known, and consequently a prediction can be calculated by the microprocessor using the calibration algorithm.
Calibration, i.e. the development and calculation of a calibration algorithm, is generally performed on a primary instrument or device when it is initialized or installed or when any of its components are replaced. This primary instrument is often a member of a group of similar instruments produced by the same manufacturer, and having the same component types, model number, and so on. The other members of this instrument group are hereinafter referred to as xe2x80x9csecondaryxe2x80x9d or xe2x80x9ctargetxe2x80x9d instruments. Because calibration is a lengthy and involved process, it is often not practical to individually recalibrate each secondary instrument in a set since this may require, among other difficulties, a large number of calibration samples at the site of each secondary instrument. Instead, for many spectroscopic applications, the calibration model developed for the primary instrument is transferred to each secondary instrument. For example, the transfer of calibration algorithms between primary and secondary instruments is often desirable with NIR spectrophotometers.
However, when a calibration model determined using measurements on a primary instrument is transferred to another instrument of the same type, a loss of accuracy generally occurs. This loss of accuracy is due to the innate differences that exist between any two physical devices, resulting in a variation in spectral responses (i.e. the correlation between pixel locations and wavelength) and affecting the reproducibility of measurements with the secondary device.
Wavelength calibration is performed to reduce the inaccuracy inherent in calibration algorithm transfer between instruments. For this purpose, calibration light source(s) having spectral lines at two or more known wavelengths are typically used to provide wavelength calibration parameters. A calibration light source may be a laser or a mercury lamp, for example. The pixel locations of the spectral lines of the calibration light source are more accurately fitted to the known wavelengths to improve the calibration reference. The calibration model or algorithm can then be adjusted to assign a more accurate wavelength value to each pixel of the multi-pixel detector array.
However, when using a calibration light source such as a laser to locate a selected wavelength on the pixel array of a spectrometer, wavelength calibration to within xc2x10.2 nm is typically very difficult to achieve, and more accurate wavelength calibration is generally not possible. For many spectroscopic applications, this level of wavelength inaccuracy remains unacceptable, since it can introduce significant errors in analyte concentration predictions or estimations, particularly for measurements performed within complex media such as living issue or blood.
Similarly, it may also be desirable to transfer a calibration developed for a primary instrument to a target instrument, where the target instrument is the primary instrument after a period of time has elapsed. During that time, the primary instrument""s response may have changed due to detector instability, temperature variations, drift in the electronics of the primary instrument, or other causes. Consequently, the primary instrument may require subsequent wavelength recalibration to avoid significant inaccuracies.
Thus, wavelength calibration of a pixel (e.g. photodiode) array spectrometer and adjustment of the calibration model to minimize differences between instruments is highly desirable, and there is a need for an improved method of wavelength calibration to facilitate and improve the transfer of a calibration algorithm or model from a primary instrument to a secondary instrument (which may include the primary instrument at a later time). The ability to accurately transfer calibration algorithms from a primary device to a secondary device is especially desirable in the case of NIR spectrophotometers, for example.
The present inventors have developed a method in order to provide an improved and more accurate method of wavelength calibration to facilitate transfer of a calibration algorithm or model from a primary instrument to a secondary or target instrument (including to the primary instrument at a later time).
In one aspect, the present invention provides a method of calibrating the wavelength of a target instrument, said target instrument being provided with a calibration model developed for a primary instrument, the method comprising:
(a) obtaining a reference set of at least two wavelength calibration parameters for the primary instrument;
(b) obtaining a target set of at least two corresponding wavelength calibration parameters for the target instrument;
(c) measuring a reference spectral response of a sample with the primary instrument;
(d) measuring a target spectral response of said sample with the target instrument;
(e) iteratively adjusting the target set of wavelength calibration parameters;
(f) for each target set of wavelength calibration parameters, determining spectral residuals corresponding to that target set; and
(g) selecting an optimum set of wavelength calibration parameters based on the spectral residuals for each target set of wavelength calibration parameters.
Preferably, step (c) comprises measuring wavelengths of the reference spectral response on a reference detector in said primary instrument and step (d) comprises measuring wavelengths of the target spectral response on a target detector in said target instrument.
More preferably, each of said reference detector and said target detector is a photodiode array detector, and each of said photodiode array detectors comprises a plurality of pixels for detecting said wavelengths.
In an embodiment, step (a) comprises generating a reference wavetable from the reference set of wavelength calibration parameters for the primary instrument, and step (b) comprises generating an initial target wavetable from the target set of wavelength calibration parameters for the target instrument, each of said wavetables providing a correlation between the pixel locations and the corresponding wavelengths in each respective instrument.
The method may further comprise: interpolating the target spectral response measured by the target instrument on to the reference wavetable; and measuring a spectral difference between the target spectral response and the reference spectral response.
The method may further comprise: interpolating the spectral difference back to the target wavetable; and subtracting the spectral difference from the initial target spectral response, so as to provide a photometrically corrected target spectral response.
The method may further comprise: interpolating the photometrically corrected target spectral response from the initial target wavetable to the reference wavetable to provide a corrected set of measured spectral data for the target instrument.
In an alternative embodiment, the method may further comprise: interpolating both the target spectral response measured by the target instrument and the reference spectral response measured by the reference instrument on to a selected wavetable; and measuring a spectral difference between the target spectral response and the reference spectral response in said selected wavetable.
This alternative method may further comprise: interpolating both the spectral difference and the initial target spectral response on to a selected wavetable; and subtracting the spectral difference from the initial target spectral response, so as to provide a photometrically corrected target spectral response.
This alternative method may further comprise: interpolating both the photometrically corrected target spectral response and the reference spectral response from the reference table on to a selected wavetable, so as to provide a corrected set of measured spectral data for the target instrument.
In each method, providing a corrected set of measured spectral data for the target instrument is repeated for each iterative adjustment in step (e).
Preferably, the iterative adjustment in step (e) comprises step-wise adjusting the pixel locations of the target detector in predetermined steps.
More preferably, step (g) comprises minimizing a parameter representative of spectral residuals of the target instrument, the spectral residuals being the difference between the corrected set of measured spectral data and an estimated set of spectral data for the target instrument.
More preferably, the parameter representative of the spectral residuals is the sum of the absolute values of all residual components across a spectrum of interest.
More preferably, the spectral residuals are calculated, during each iteration, using one of principle components regression, principal components analysis, and partial least squares.
In an alternative embodiment, the parameter representative of the spectral residuals is one of the mean, median, and root mean square of the absolute values of at least some residual components.
Preferably, the spectral residuals are calculated, during each iteration, using one of principle components regression, principal components analysis, and partial least squares.
More preferably, said iterative adjustment in step (e) comprises step-wise adjusting the pixel locations of the target detector in predetermined steps.
In another aspect, the present invention provides a method of recalibrating the wavelength of a target instrument, said target instrument being provided with a calibration model developed for a primary instrument, the method comprising:
(a) obtaining a reference set of at least two wavelength calibration parameters for the primary instrument,
(b) obtaining a target set of at least two corresponding wavelength calibration parameters for the target instrument;
(c) measuring a reference spectral response of a sample with the primary instrument;
(d) measuring a target spectral response of said sample with the target instrument;
(e) iteratively adjusting the target set of wavelength calibration parameters;
(f) for each target set of wavelength calibration parameters, determining spectral residuals corresponding to that target set; and
(g) selecting an optimum set of wavelength calibration parameters based on the spectral residuals for each target set of wavelength calibration parameters.
In a preferred embodiment, steps (b) and (d) occur after steps (a) and (c), and the target instrument to be recalibrated is the primary instrument itself.
The objects and advantages of the present invention will be better understood and more readily apparent with reference to the remainder of the description in conjunction with the accompanying drawings.